It is useful to be able to predict the extent to w

问题补充：

It is useful to be able to predict the extent to which a price change will affect supplyand demand.

答案：

it:形式主语；

is系动词,作谓语的一部分；

useful表语,作谓语的一部分；

to be able to predict the extent to which a price change will affect supply：真正的主语.

真正的主语是一个不定式短语,其中the extent to which a price change will affect suppl是predict的宾语,这个宾以extent为中心词,to which a price change will affect supply是定语从句,修饰extent.

在这个定语从句中,a price change是主语,will affect是谓语；supply是宾语.